Abstract-The n.m.r. spectra of arecoline, hordenine, strychnine, brucine and their salts were determined and the chemical shifts reported previously for arecoline have been verified by single frequency proton decoupling techniques. Effects of protonation on the carbon resonances in aiding unambiguous assignments in strychnine and brucine are discussed. The quaternary N-methiodide of brucine was also studied to confirm the assignments made for carbon atoms adjacent to the basic N atom.THE APPLICATION of 13C n.m.r. spectroscopy to structural studies of natural products is well-known. Among these compounds alkaloids have received considerable attention;1-5 in particular, Wenkert and co-workers have not only assigned the resonances of many classes of alkaloids, but have provided significant guidelines for efficient use of 13Cn.m.r. in these kinds of structural studies. This subject has been reviewed extensive1y.l Work in progress in our laboratory is directed towards the application of 15N n.m.r. to structure elucidation of alkaloids and we wish to correlate and compare our results with corresponding 13C data. Perusal of the literature revealed that the 13C spectra of arecoline, hordenine, strychnine and brucine (1-4) have not been completely reported, but only a partial assignment of strychnine (3), based on T, values, exists6 Furthermore, we wish to confirm the reported2 assignments of C-2 and C-6 in arecoline (l), which were based on assumed substituent effects. Although brucine is a minor structural analog of strychnine, assignments of both serve as mutual confirmations. EXPERIMENTALProton noise-decoupled or off-resonance decoupled Fourier transform 13C spectra were obtained at 25.03 MHz on a JEOL PS/PFT-100 n.m.r. spectrometer equipped with the JEOL EC-100 data system. From 250 to 2500 transients were accumulated for proton noise-decoupled spectra over a 5 KHz range using 8 K words of memory (1.22 Hzladdress); 1000-15000 transients were accumulated for proton off-resonance decoupled spectra. A 30' pulse was employed at repetition rates ranging from 1.5-2.0 s depending upon the sample. All compounds were commercial samples and were used without further purification. CDCI, solutions in 10 mm 0.d. tubes were employed for the free alkaloids as well as for strychnine + trifluoroacetic acid (TFA, 1:1) and brucine t TFA (l:l), using TMS as internal reference. The RESULTS AND DISCUSSION Arecoline and arecoline hydrobromideAlthough the 13C spectrum of arecoline had been reported earlier,2 we re-examined it and its hydrobromide (la) in order to assign unambiguously the resonances of C-2 and C-6. In addition, we found analysis of the shifts of the olefinic carbons upon protonation of (1) to be helpful in assignment of the olefinic carbons in strychnine and brucine and their TFA derivatives.The proton noise-decoupled spectrum of 1 (CDCI,) showed eight resonances at 26-68(t), 45*70(q), 51 -4O(q), 51*40(t), 53*30(t), 128. and q denote the multiplicity of the signal as singlet, doublet, triplet and quartet, respectively, in t...
It has been found possible to measure the shifts of a variety of amines to an accuracy of 0.2 ppm with the isotope at its natural-abundance level. Successful observation of these resonances with our existing equipment is dependent on a favorable nuclear Overhauser effect arising from irradiation of the protons. Chemical exchange of protons directly bound to the amine nitrogen can reduce the signal intensity sufficiently to make detection of the 15N resonance difficult or impossible. The 15N shifts generally correlate linearly with the 13C shifts of alkanes derived by replacing the amine nitrogen with an appropriately substituted carbon. Detailed examination leads to deshielding a and ß substituent shift parameters, a shielding y effect, and possible deshielding associated with hydrogen bonding. Effects of constraints in geometrically rigid systems as well as incorporation of the nitrogen into six-, five-, and three-membered rings are discussed and compared with corresponding 13C shifts. Parallel trends were found for all cases. Natural-abundance 15N spectroscopy is thus a practicable, if far from routine, means of structural analysis. Recentdevelopments in nmr instrumentation have . greatly facilitated the application of 13C spectroscopy to problems in organic chemistry.2 Characteristic functional group chemical shifts, well-defined substituent and geometrical parameters, the virtual absence of 13C-13C spin-spin coupling in natural abundance, and the favorable Overhauser effect and spectral simplicity associated with proton noise-decoupling to eliminate 13€spin-spin interactions have all contributed to the success which 13C nmr spectroscopy now enjoys. Fourier-transform spectroscopy can be expected to further expand areas of cmr investigations, especially in the biochemical realm. The relatively small use of nitrogen nmr spectroscopy stands in sharp contrast. Even though involved in one of the first applications of nmr to structure elucidation,3 456the spectroscopy of 14N and 15N has nevertheless been relatively little exploited. The reasons for this are straightforward: nitrogen-14, while the dominant isotope (natural abundance = 99.64%), has a large nuclear quadrupole moment associated with its spin quantum number (/ = 1) which may provide an efficient relaxation mechanism and, when operative, usually results in both large line widths and substantial errors in chemical-shift measurements. Line widths of over 1000 Hz and uncertainties of up to 60 ppm have been reported for 14N resonances.4,5 As a result, subtle differences in chemical shifts of structurally related compounds are generally undetectable, and only gross correlations between structure and chemical shift have been possible.4-12 As a further consequence of quad-(1) (a)
The 13C chemical shift of chloroform has been measured in a variety of solvents. Relative to dilute solution in cyclohexane, all of the solvents studied resulted in downfield shifts which, with the exceptions of benzene and acetonitrile, correlate linearly (r = 0.982) with the changes in proton shifts in the same solvents. The results, taken with the variation of the 18C-H coupling constant with solvent, suggest that the solvent effects arise from changes in the average distance of the bonding electrons in the chloroform C-H bond as the result of intermolecular association. Contributions from the anisotropy of benzene and acetonitrile indicate that measurements of the chemical shift of more than one nucleus in the same molecule may allow detection and estimation of neighbor anisotropy contributions to chemical shifts. (1963).
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